Latest / Daniel and Kelly’s Extraordinary Universe / Listener questions #53

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Listener questions #53 · Daniel and Kelly’s Extraordinary Universe

• 0:07
  Speaker 1: I heard the LHC made a heavy proton out of
• 0:10
  Speaker 1: thin air. But what does it mean and why should
• 0:13
  Speaker 1: I care?
• 0:14
  Speaker 2: You smash particles together and they go boom. But how
• 0:18
  Speaker 2: does their data get to the computer room?
• 0:21
  Speaker 1: I really want to understand desider space. Can you explain
• 0:25
  Speaker 1: it so I get a big smile on my face?
• 0:27
  Speaker 2: Whatever questions keep you up at night, Daniel and Kelly's
• 0:30
  Speaker 2: answers will make it right.
• 0:32
  Speaker 1: Welcome to Daniel and Kelly's Extraordinary Universe, Listener Questions, All
• 0:37
  Speaker 1: Physics Edition.
• 0:38
  Speaker 3: 53! Hello, I'm Kelly Wiedersmith.
• 0:54
  Speaker 2: I study parasites and space, and we're talking about a
• 0:58
  Speaker 2: lot of particles today. And I'm pretty excited because we've got,
• 1:00
  Speaker 2: you know, Daniel, the particle physicist. And so, Daniel, what
• 1:03
  Speaker 2: I want to know is, and I'll go ahead and
• 1:05
  Speaker 2: introduce you. My co-host is Daniel Weitz, and he thinks
• 1:07
  Speaker 2: about aliens all the time, but sometimes he also thinks
• 1:09
  Speaker 2: about particles.
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  Speaker 3: And so, Daniel.
• 1:12
  Speaker 1: That's my day job.
• 1:14
  Speaker 3: What is your favorite particle?
• 1:16
  Speaker 1: Oh man, I don't have a favorite particle. What I
• 1:18
  Speaker 1: love is the variety of particles, the diversity, the weirdness,
• 1:23
  Speaker 1: the patterns. Because it's in those patterns that are the
• 1:27
  Speaker 1: clues to understanding the next layer of reality. The way
• 1:31
  Speaker 1: that you need all the elements in the periodic table
• 1:34
  Speaker 1: to help you understand that all the weird chemistry comes
• 1:37
  Speaker 1: from the structure of the atom, how the protons and
• 1:40
  Speaker 1: neutrons and electrons all click together. In the same way,
• 1:43
  Speaker 1: the weirdness of the current set of particles is a
• 1:47
  Speaker 1: clue as to what's going on underneath. We just don't
• 1:50
  Speaker 1: know it yet. And so I love looking at it
• 1:52
  Speaker 1: and imagining some physicist in 100 years who knows that
• 1:55
  Speaker 1: answer and is saying like, wow, all the clues were
• 1:58
  Speaker 1: right there. Anybody in 2026 could have won a Nobel
• 2:02
  Speaker 1: Prize if only they were smart enough.
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  Speaker 4: Nope.
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  Speaker 3: What's your favorite particle?
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  Speaker 2: That was uplifting or whatever, but that wasn't the answer
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  Speaker 2: to the question.
• 2:13
  Speaker 1: All right. Well, I got my PhD on the top cork,
• 2:15
  Speaker 1: so then I got.
• 2:16
  Speaker 3: To go for the top cork.
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  Speaker 1: Yeah, that's the one.
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  Speaker 3: Thank you.
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  Speaker 1: I spent a lot of time with the top cork.
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  Speaker 1: It was back when we had a small amount of data.
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  Speaker 1: And so I had like seven collisions on which I
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  Speaker 1: wrote my PhD and I knew all of them by name. So, yeah.
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  Speaker 3: Amazing. OK, there you go.
• 2:34
  Speaker 2: Because, you know, I was wondering, like, is Daniel going
• 2:35
  Speaker 2: to go into politics next? I can't let him get
• 2:38
  Speaker 2: away with evading my questions.
• 2:40
  Speaker 3: So anyway.
• 2:41
  Speaker 1: All right.
• 2:41
  Speaker 3: Now we're good.
• 2:42
  Speaker 2: But now let's get to some more heavy hitting questions.
• 2:45
  Speaker 2: The questions from our listeners.
• 2:47
  Speaker 4: All right.
• 2:47
  Speaker 2: So let's start with our first particle question of the
• 2:50
  Speaker 2: day from Catherine.
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  Speaker 4: Hi, Daniel and Kelly. I just heard about a new
• 2:55
  Speaker 4: particle that was discovered at the Large Hadron Collider. They're
• 3:00
  Speaker 4: calling it a heavy proton. I'd like you to comment
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  Speaker 4: about the importance of this discovery. And also, if it
• 3:08
  Speaker 4: only lasts for a billionth, trillionth of a second, does
• 3:13
  Speaker 4: it really have any value? All right.
• 3:16
  Speaker 1: I love how she starts out with the science and
• 3:19
  Speaker 1: then she's like, What's it worth, man?
• 3:22
  Speaker 2: I mean, I think that's a totally great question. And actually,
• 3:25
  Speaker 2: I got to say, I must be living in Mars
• 3:28
  Speaker 2: in a cave or something. I didn't hear that we
• 3:30
  Speaker 2: just discovered a new particle at the LHC. I mean,
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  Speaker 2: is that Nobel Prize level discovery here?
• 3:37
  Speaker 1: It's not really. And the particle physicists who work on
• 3:40
  Speaker 1: this kind of stuff are going to hate me for
• 3:42
  Speaker 1: saying this, but this is like the chemistry side of
• 3:45
  Speaker 1: particle physics. And it's not really that earth shattering of
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  Speaker 1: a discovery. I mean, it's cool. And I'm glad they
• 3:53
  Speaker 1: did it. And we definitely learned something. But for people
• 3:56
  Speaker 1: who were like, hey, they got a Nobel Prize for
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  Speaker 1: the Higgs. Why isn't this making the same kind of
• 4:00
  Speaker 1: earthquake in particle land? There's a reason for that.
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  Speaker 3: I like how you threw shade on two communities at once.
• 4:06
  Speaker 2: You're like a bunch of physicists aren't going to like me,
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  Speaker 2: but it's boring because it's the chemistry version.
• 4:10
  Speaker 3: And I was like, whoa, Daniel's hitting out at everyone today.
• 4:13
  Speaker 4: Yeah.
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  Speaker 1: Well, the difference is fundamental versus composite particles. So fundamental
• 4:20
  Speaker 1: particles are things that we don't know that they're made
• 4:23
  Speaker 1: of anything smaller. They might just be made of themselves,
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  Speaker 1: like the electron. We don't know that there's anything inside
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  Speaker 1: the electron. There might be. It might be that a
• 4:31
  Speaker 1: future collider tells us what the electron is made out of.
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  Speaker 1: And it's all made of squiglions underneath. And that explains
• 4:37
  Speaker 1: all the amazing patterns. But so far, we can't see that.
• 4:41
  Speaker 1: And so we treat the electron as if it's fundamental.
• 4:44
  Speaker 1: Same for the top quark and the other quarks and
• 4:46
  Speaker 1: the force particles like the photon. So we have this
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  Speaker 1: set of fundamental particles that we include in our table
• 4:52
  Speaker 1: of fundamental particles. And so you can think of those
• 4:55
  Speaker 1: sort of like atoms of particle physics, right? The indivisible
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  Speaker 1: building blocks out of which everything is built. The other
• 5:02
  Speaker 1: kind of particle is a composite particle. It means it's
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  Speaker 1: made of other stuff. So for example, the proton, it's
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  Speaker 1: made out of three quarks, two up quarks and a down.
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  Speaker 1: Or the neutron is one up and two downs. And
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  Speaker 1: there's a zillion different kinds of these composite particles because
• 5:18
  Speaker 1: there's a zillion different ways to click together quarks to
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  Speaker 1: make them. You can do pairs of quarks like a pion,
• 5:23
  Speaker 1: different pairs of quarks, different triplets of quarks. So you
• 5:26
  Speaker 1: can combine these basic building blocks of fundamental particles to
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  Speaker 1: make lots of different composite particles. So in the particle
• 5:33
  Speaker 1: physics land, these are more like molecules than atoms. They're
• 5:36
  Speaker 1: made of fundamental bits, but they're just rearrangements of those bits.
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  Speaker 2: And so it sounds like you're saying that's what happened
• 5:43
  Speaker 2: at the LHC recently, is we clicked together bits in
• 5:45
  Speaker 2: a new combination? Yeah.
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  Speaker 1: And so if you discover something like the Higgs boson
• 5:51
  Speaker 1: or the top quark, or the charm quirk, or the Tao,
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  Speaker 1: or something that's new and fundamental, that's a big deal
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  Speaker 1: because it's like discovering a new atom. And it tells
• 6:00
  Speaker 1: you about what the universe itself can do. It gives
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  Speaker 1: you a clue as to either the fundamental nature of
• 6:06
  Speaker 1: the universe, if these really are fundamental, or the next
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  Speaker 1: layer down, the squiglions that make up what we think
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  Speaker 1: is fundamental. So that's really cool. That's really insightful. That's
• 6:17
  Speaker 1: an earthquake in physics when it happens, right? But it
• 6:20
  Speaker 1: doesn't happen very often. You know, we found the Higgs
• 6:23
  Speaker 1: in 2012. We found the top quark in 95, right?
• 6:27
  Speaker 1: Before that, it was the bottom quark in 77 and
• 6:29
  Speaker 1: a few neutrinos here and there. And so it's a
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  Speaker 1: rare thing. It happens every couple of decades. It's almost
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  Speaker 1: always a Nobel Prize because it gives you really deep
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  Speaker 1: insight either into the nature of space-time itself or into
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  Speaker 1: the next layer of reality. So really a big deal.
• 6:44
  Speaker 1: But when you discover a new composite particle, it's just
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  Speaker 1: like you say, you're clicking together the stuff you already
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  Speaker 1: know in a new way. And that's not without value, right?
• 6:53
  Speaker 1: In the same way that like when you build a
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  Speaker 1: new kind of structure in chemistry, you're learning about the
• 6:58
  Speaker 1: kind of structures you can make in chemistry. Oh, it
• 7:00
  Speaker 1: turns out you can make this kind of molecule and
• 7:02
  Speaker 1: this is stable and whatever. And so that's what's happened
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  Speaker 1: here is it tells you what the fundamental particles can do.
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  Speaker 1: So this is like doing chemistry with the atoms of
• 7:12
  Speaker 1: fundamental physics, right? Which are now the fundamental particles. What
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  Speaker 1: they did was they found a particle that was made
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  Speaker 1: of two charm quarks and a down quark. So remember,
• 7:21
  Speaker 1: the proton is two up quarks and a down quark.
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  Speaker 1: And it's stable. It lasts forever. You have a proton.
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  Speaker 1: You leave it in space. You come back a billion
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  Speaker 1: years later. It'll still be there. We've never seen a
• 7:33
  Speaker 1: proton decay because it's made of the lightest quarks. It
• 7:37
  Speaker 1: just sits there. And neutrons are not stable because there
• 7:40
  Speaker 1: are a little bit more energy stored in them And
• 7:43
  Speaker 1: so they can decay into a proton, but a proton
• 7:45
  Speaker 1: is stable. But you can click together other stuff, right?
• 7:48
  Speaker 1: You can replace those up quarks with a charm quark,
• 7:51
  Speaker 1: because remember, the charm quark is just like the up quark,
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  Speaker 1: but it has more mass. So two charms and a down,
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  Speaker 1: which is this new particle they found, is very similar
• 8:01
  Speaker 1: to ups and a down, which is the proton. So
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  Speaker 1: you can call this in popular science language, a heavy proton.
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  Speaker 1: But in physics language, it has its own name, which
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  Speaker 1: we don't need to get into because I didn't tell
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  Speaker 1: you anything, but it's some new particle.
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  Speaker 2: And so does the heavy proton, is this less stable
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  Speaker 2: because it has more mass than the normal proton? And
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  Speaker 2: then what else do we learn by knowing that this exists?
• 8:26
  Speaker 1: Yeah, you're exactly right. It's very unstable. It doesn't last
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  Speaker 1: for very long. It doesn't like to stick around. It
• 8:32
  Speaker 1: decays very quickly. And so what do we learn? We
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  Speaker 1: learn about how these particles click together, which is very
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  Speaker 1: interesting because the thing that determines what particles can click
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  Speaker 1: together and how long they live is the strong nuclear force. Remember,
• 8:45
  Speaker 1: quarks have an extra kind of charge. Electrons have an
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  Speaker 1: electric charge, but quarks have an electric charge and a
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  Speaker 1: color charge, which means that they participate in the strong
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  Speaker 1: nuclear force and they pass gluons around. And that force
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  Speaker 1: is very, very strong, which makes it very hard to
• 9:01
  Speaker 1: calculate What will happen? And so it's difficult to know
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  Speaker 1: what particles can form. How long do they live? Which
• 9:08
  Speaker 1: ones are more stable? Which ones are less stable? These
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  Speaker 1: calculations are very difficult because our calculational tools work best
• 9:16
  Speaker 1: for weaker forces because we can't do any of these
• 9:19
  Speaker 1: calculations straight up exactly. We have to do approximations where
• 9:22
  Speaker 1: we break up the calculation into pieces and ignore a
• 9:25
  Speaker 1: bunch of the ones that we can't do. And when
• 9:28
  Speaker 1: you're dealing with a strong force, there are fewer that
• 9:30
  Speaker 1: you can ignore because the force is so strong. So
• 9:33
  Speaker 1: there's a huge community of folks calculating these things and
• 9:36
  Speaker 1: trying to predict it. It's big. It's complicated. The sort
• 9:39
  Speaker 1: of the way chemistry is complicated, like it's hard to
• 9:42
  Speaker 1: know in advance what molecules you can make and what
• 9:45
  Speaker 1: they're going to do. And so in the same way,
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  Speaker 1: when you make a new particle, a new composite particle,
• 9:51
  Speaker 1: you learn something about what the strong force can do
• 9:54
  Speaker 1: and whether your calculations are correct. And that's really valuable.
• 9:58
  Speaker 1: And we've done a bunch of it at the LHC.
• 10:00
  Speaker 1: Like we haven't found a new fundamental particle since 2012,
• 10:03
  Speaker 1: the Higgs boson, but we found literally dozens of these
• 10:07
  Speaker 1: composite particles. Last time I checked the count, it was
• 10:10
  Speaker 1: more than 50. Wow. And so that's why they're not
• 10:13
  Speaker 1: awarding a new Nobel prize for each one, but they
• 10:16
  Speaker 1: do tell you something about the strong force and how
• 10:18
  Speaker 1: it clicks together and how our universe works. And so
• 10:21
  Speaker 1: it is very valuable. And I'm probably biased because it's
• 10:24
  Speaker 1: not the kind of thing that I study. So it's
• 10:26
  Speaker 1: not the kind of thing that I'm particularly excited about,
• 10:28
  Speaker 1: which is discovering a new fundamental particle. That doesn't mean,
• 10:31
  Speaker 1: of course, it's without value. It just means it's, you know,
• 10:35
  Speaker 1: a value to a different community and for different reasons.
• 10:38
  Speaker 3: All right. Well, that was illuminating. Let's see what Catherine
• 10:41
  Speaker 3: has to say.
• 10:42
  Speaker 4: Thank you, Daniel and Kelly. I appreciate your clear explanation
• 10:47
  Speaker 4: of a heavy proton. If I could use a cooking analogy,
• 10:52
  Speaker 4: it's rather like taking an old dish and tweaking the
• 10:56
  Speaker 4: ingredients rather than coming up with a completely new dish.
• 11:01
  Speaker 4: As a retired art teacher and a non-science major, I
• 11:06
  Speaker 4: appreciate your making science accessible to me. Keep up the
• 11:11
  Speaker 4: good work.
• 11:32
  Speaker 3: And we're back. And the next question is a CERN
• 11:34
  Speaker 3: question from Chris.
• 11:36
  Speaker 5: Hi, Daniel and Kelly. I feel like I have a
• 11:39
  Speaker 5: decent grasp of how particle colliders get the particles up
• 11:42
  Speaker 5: to high velocities and smash them into each other. I
• 11:45
  Speaker 5: think that can be fairly summarized as magnets. You've discussed
• 11:50
  Speaker 5: that a few times on the show, but something I'm
• 11:52
  Speaker 5: in the dark about is how CERN and other projects
• 11:55
  Speaker 5: collect so much data about these infinitesimal little particles that
• 11:58
  Speaker 5: come out of the collisions. How do we determine the
• 12:01
  Speaker 5: mass and the velocity and the direction of all these particles?
• 12:04
  Speaker 5: What sort of materials do we use to collect the
• 12:06
  Speaker 5: particles and How do we get that information into computers?
• 12:10
  Speaker 5: How can we tell a proton from a muon from
• 12:12
  Speaker 5: a Higgs? How do we know if they all came
• 12:14
  Speaker 5: from the same collision? I would love to hear you
• 12:16
  Speaker 5: talk about that in some detail. That'd be great.
• 12:19
  Speaker 1: Thank you so much.
• 12:20
  Speaker 3: All right, Daniel, how do we make these measurements?
• 12:22
  Speaker 1: Yeah, thanks for this question, Chris. I love talking about
• 12:25
  Speaker 1: this stuff. People have asked a version of this a
• 12:27
  Speaker 1: few times, but there's always more to say about how
• 12:30
  Speaker 1: to see particles. And so the basic version of the
• 12:34
  Speaker 1: story is that you're right. We have the collider. And
• 12:37
  Speaker 1: we use magnets and little linear accelerators to make things
• 12:40
  Speaker 1: go fast. So the way the accelerator works is you
• 12:43
  Speaker 1: have a little linear accelerator, which is just like a
• 12:45
  Speaker 1: big voltage difference. Like you put an electric field and
• 12:48
  Speaker 1: it pulls particles with the charge and makes them go faster.
• 12:52
  Speaker 1: And then you bend them with a magnet. And then
• 12:54
  Speaker 1: you push them with a field. And then you bend
• 12:56
  Speaker 1: them with a magnet. So it's push and bend, push
• 12:58
  Speaker 1: and bend, push and bend all the way around the ring.
• 13:00
  Speaker 1: Now you have your particles going really, really fast. But
• 13:03
  Speaker 1: you actually have two colliders. And two colliders are going
• 13:06
  Speaker 1: in different directions around the ring. So you have these
• 13:08
  Speaker 1: two beams circling at insane speeds around the ring opposite
• 13:13
  Speaker 1: each other. And then you cross them. So you collide
• 13:16
  Speaker 1: the beams. And the beams are not just like a
• 13:19
  Speaker 1: big stream of particles. They're organized in bunches. That's the
• 13:23
  Speaker 1: technical term. See, we don't use some ridiculous Latin phrase.
• 13:27
  Speaker 1: And each bunch is like 10 to the 12 or
• 13:30
  Speaker 1: 10 to the 13 protons. And they pass through another
• 13:34
  Speaker 1: bunch going the other direction. So you have these bunches
• 13:37
  Speaker 1: and they cross at this collision point. And then the
• 13:40
  Speaker 1: next bunch crosses and the next bunch crosses.
• 13:43
  Speaker 2: When you cross the beams, do you then use them
• 13:45
  Speaker 2: to like send ghosts back to the portal that they
• 13:47
  Speaker 2: came from?
• 13:47
  Speaker 1: You're propagating misinformation.
• 13:52
  Speaker 2: Oh, no, I was thinking about the Ghostbusters where they say,
• 13:56
  Speaker 2: don't cross the beams. I watched the original Ghostbusters movie
• 14:00
  Speaker 2: with my daughter recently and A, was surprised by how
• 14:02
  Speaker 2: much sexism I had forgot about was in that. But
• 14:05
  Speaker 2: the don't cross the beams is in my head right now.
• 14:08
  Speaker 2: So anyway, no ghosts being sent back to another portal
• 14:11
  Speaker 2: at CERN.
• 14:11
  Speaker 1: We are not opening portals at CERN. It's not happening. No,
• 14:16
  Speaker 1: what happens when you cross the beam is sometimes the
• 14:18
  Speaker 1: particles collide. And sometimes you get no protons colliding because
• 14:23
  Speaker 1: it's really hard to get tiny protons to hit each other.
• 14:25
  Speaker 1: Mostly they just pass through, which is why you have
• 14:27
  Speaker 1: like 10 to the 12 in a bunch. Because sometimes
• 14:30
  Speaker 1: you get zero collisions, sometimes you get 10, sometimes you
• 14:33
  Speaker 1: get 200 collisions. And the job of the LHC engineers
• 14:36
  Speaker 1: is to make more collisions happen so we can see
• 14:39
  Speaker 1: more stuff and we can see rarer and rarer stuff.
• 14:42
  Speaker 4: Okay.
• 14:43
  Speaker 2: And so once they collide, how do you know that
• 14:47
  Speaker 2: that has happened?
• 14:48
  Speaker 3: You can't see it.
• 14:49
  Speaker 1: You can't see the collision itself. You're absolutely right. What
• 14:53
  Speaker 1: we can see is the debris from the collision. So
• 14:56
  Speaker 1: it's like a car crash happened and we're coming in
• 14:58
  Speaker 1: afterwards and And we're picking up the pieces that were
• 15:00
  Speaker 1: like thrown onto the sidewalk.
• 15:02
  Speaker 3: That's dark.
• 15:02
  Speaker 1: And so the protons collide. And then something happens because
• 15:08
  Speaker 1: the quarks inside them interact. So the protons don't actually collide.
• 15:12
  Speaker 1: At those energies, the bonds between the quarks have low
• 15:16
  Speaker 1: energy compared to the velocity of the quarks. And so
• 15:19
  Speaker 1: the fact that they're in a proton together becomes kind
• 15:21
  Speaker 1: of irrelevant. And you just have like a pile of
• 15:23
  Speaker 1: quarks hitting another pile of quarks. And then maybe those
• 15:26
  Speaker 1: quarks will interact. So you have like a quark from
• 15:29
  Speaker 1: one proton interacting with a quark from the other proton,
• 15:32
  Speaker 1: and then they do something. Last episode, we talked about
• 15:34
  Speaker 1: annihilation or interaction. So the quarks can annihilate with each
• 15:38
  Speaker 1: other or they can just turn into something else. So
• 15:41
  Speaker 1: all sorts of stuff can happen. And the most common
• 15:44
  Speaker 1: thing is that it's just like a quark bounces off
• 15:46
  Speaker 1: quark and they go off their separate ways and everybody
• 15:48
  Speaker 1: reforms into particles and it's very boring. And most of
• 15:51
  Speaker 1: it just goes down the bean pipe. That's the most
• 15:53
  Speaker 1: common thing to happen. Basically nothing interesting. But sometimes they
• 15:57
  Speaker 1: make like a weird heavy particle, maybe a Z or
• 16:00
  Speaker 1: a top or a Higgs or something. And then that
• 16:03
  Speaker 1: doesn't last for very long, like 10 to the minus
• 16:05
  Speaker 1: 20 seconds. So you're right, you can't see it. But
• 16:08
  Speaker 1: it decays and it sprays into a huge stream of
• 16:11
  Speaker 1: other particles. And those are the ones that we can see.
• 16:14
  Speaker 1: So it throws muons or electrons or neutrons or protons
• 16:18
  Speaker 1: or something, a bunch of stuff out the sides. And
• 16:21
  Speaker 1: then we build detectors around the collision point, basically like
• 16:24
  Speaker 1: camera's to capture those and to then infer from the
• 16:28
  Speaker 1: patterns of those particles what's going on. And that's basically
• 16:32
  Speaker 1: my job. That's the piece I work on. You know,
• 16:35
  Speaker 1: it's a huge complex, people who built the accelerator, people
• 16:37
  Speaker 1: who make it work, people who built the detectors, people
• 16:40
  Speaker 1: who work on like keeping the detectors working and checking
• 16:43
  Speaker 1: that they're working and maintaining it and replacing bits, all
• 16:46
  Speaker 1: sorts of stuff. The bit that I'm most interested in
• 16:49
  Speaker 1: is the bit where you do the statistical analysis to
• 16:51
  Speaker 1: try to figure out like what happened there. And how
• 16:54
  Speaker 1: do we develop new statistical tools to get the most
• 16:56
  Speaker 1: information out of that? Because it's a complicated statistical problem
• 17:00
  Speaker 1: because you have really high dimensional information. We take 100
• 17:04
  Speaker 1: million measurements of every particle collision in various ways. And
• 17:08
  Speaker 1: we have to use that information to figure out, was
• 17:10
  Speaker 1: this dark matter? Was it just two protons bouncing off
• 17:13
  Speaker 1: each other? What was going on? And so the detector
• 17:16
  Speaker 1: is built out of several layers to help us do that.
• 17:19
  Speaker 3: So every time.
• 17:21
  Speaker 2: a new particle is made, you get a hundred million
• 17:23
  Speaker 2: measurements of it?
• 17:25
  Speaker 1: Every single time we collide protons, we get a hundred
• 17:28
  Speaker 1: million measurements of the collision.
• 17:30
  Speaker 4: Yeah.
• 17:31
  Speaker 3: Got it.
• 17:31
  Speaker 4: Okay.
• 17:31
  Speaker 2: I was like, how do you measure one thing a
• 17:34
  Speaker 2: hundred million times? And is that overkill?
• 17:37
  Speaker 3: But anyway, okay.
• 17:38
  Speaker 1: It's sort of just like having a big camera, you know,
• 17:40
  Speaker 1: it's like a hundred megapixel camera, right? And it has
• 17:43
  Speaker 1: a bunch of possibilities. Often a lot of those are
• 17:45
  Speaker 1: just blank, right? Because no particle hit it. but we
• 17:48
  Speaker 1: have 100 million channels in which to see particles. And
• 17:51
  Speaker 1: we have it staggered in these various layers that surround
• 17:54
  Speaker 1: the collisions so that we can figure out, just to
• 17:57
  Speaker 1: answer Chris's question, what particle was there and how do
• 18:00
  Speaker 1: we get the information about it?
• 18:02
  Speaker 2: Okay, and so the data that's collected is essentially which
• 18:05
  Speaker 2: one of the sensors got hit. And then that's the
• 18:08
  Speaker 2: information that goes to you.
• 18:10
  Speaker 1: Yeah, exactly. And we have several different kinds of sensors.
• 18:13
  Speaker 1: Just closest around the beam itself We have these silicon-based trackers.
• 18:18
  Speaker 1: These are just wafers of silicon. And when a particle
• 18:22
  Speaker 1: goes through them, it knocks off an electron and the
• 18:25
  Speaker 1: wafer is kept under voltage so that when you knock
• 18:27
  Speaker 1: off that electron, it gets pulled towards one side and
• 18:30
  Speaker 1: you can read it out as a current. And so
• 18:32
  Speaker 1: that's how you know, oh, a particle went through this
• 18:34
  Speaker 1: wafer of silicon.
• 18:35
  Speaker 3: Clever.
• 18:35
  Speaker 1: And silicon wafers are great because you can print them
• 18:38
  Speaker 1: using silicon lithography techniques that we borrow from consumer industries
• 18:43
  Speaker 1: to be really, really small little pieces of the wafer.
• 18:45
  Speaker 1: So you can tell exactly where in the wafer it
• 18:48
  Speaker 1: hit because you've subdivided it into all these little bits.
• 18:51
  Speaker 1: So it's really high precision and we can make them
• 18:54
  Speaker 1: really thin. And so we surround the beam pipe with
• 18:56
  Speaker 1: these to get really precise measurements of where the particles went.
• 18:59
  Speaker 1: That gives you like a visual picture of like, oh,
• 19:01
  Speaker 1: a particle went through this layer and then this layer
• 19:03
  Speaker 1: and then this layer. So you can reconstruct its trajectory, right?
• 19:07
  Speaker 1: You can say, oh, it went this way. Another one
• 19:08
  Speaker 1: went to that way. So you have as many layers
• 19:11
  Speaker 1: of the silicon as you can. The downside with silicon
• 19:14
  Speaker 1: is that it's expensive and it's very fragile. Like it
• 19:17
  Speaker 1: can't take radiation for very long. It's electronics. Electronics don't
• 19:21
  Speaker 1: like radiation. So every couple of years we got to
• 19:24
  Speaker 1: rip it out and put in new ones because it
• 19:25
  Speaker 1: gets used up basically.
• 19:27
  Speaker 2: And each one of these levels that you're telling us about,
• 19:30
  Speaker 2: do they interact with particles in different ways? Or do they,
• 19:34
  Speaker 2: like for the silicon chips, no matter what hits it,
• 19:37
  Speaker 2: does it give off a signal?
• 19:39
  Speaker 1: Yeah, great question. So silicon only interacts with charged particles.
• 19:43
  Speaker 1: So a neutrino goes to the silicon, does nothing. But
• 19:46
  Speaker 1: anything that carries a charge, so like an electron, a muon,
• 19:48
  Speaker 1: a proton, these will leave a trace in your silicon.
• 19:53
  Speaker 1: But sometimes a non-trace is useful because if you see
• 19:56
  Speaker 1: like a trace downstream in a different kind of detector
• 19:59
  Speaker 1: that like can see neutrons and then no trace in
• 20:02
  Speaker 1: your silicon, That's a confirmation. Oh, that probably was a
• 20:05
  Speaker 1: neutron because it didn't leave a trace here in the silicon.
• 20:09
  Speaker 1: And that's the kind of thing we put together later.
• 20:10
  Speaker 1: We say like, okay, you have information from this layer
• 20:13
  Speaker 1: and then that layer and the other layer. And all together,
• 20:15
  Speaker 1: it'll tell you what kind of particle it was. So
• 20:18
  Speaker 1: the inner detector mostly responds to charge particles. Then the
• 20:22
  Speaker 1: whole thing is immersed in a magnetic field. So the
• 20:25
  Speaker 1: particles bend. And that lets us know the charge of
• 20:28
  Speaker 1: the particle and its momentum. Because only charged particles bend
• 20:32
  Speaker 1: in magnetic fields. And if they're charged positive, they bend
• 20:35
  Speaker 1: one way. If they're charged negative, they bend the other way.
• 20:38
  Speaker 1: So we can tell if it's a positive or negative charge.
• 20:41
  Speaker 1: And if it's going really fast, it hardly bends at all.
• 20:44
  Speaker 1: If it's going really slow, it bends a lot. So
• 20:47
  Speaker 1: by measuring that curvature, we can tell how fast was
• 20:50
  Speaker 1: it going.
• 20:51
  Speaker 2: This does sound like a pretty difficult experiment. Like statistical
• 20:54
  Speaker 2: analysis that needs to happen afterwards. There's like a lot
• 20:56
  Speaker 2: of pieces of information that need to get integrated to
• 20:58
  Speaker 2: figure out what went through there.
• 21:00
  Speaker 1: Exactly. And historically, what we've done is simplify the problem
• 21:03
  Speaker 1: by doing each of those separately. Like let's first figure
• 21:06
  Speaker 1: out the angle of the particle, then let's combine it
• 21:08
  Speaker 1: with this. And along the way, we would lose information.
• 21:11
  Speaker 1: So the trend these days is to try to do
• 21:13
  Speaker 1: end-to-end analysis of the lowest level information to slurp up
• 21:17
  Speaker 1: as much information as possible. And that used to be
• 21:20
  Speaker 1: computationally intractable, but now thanks to AI and machine learning,
• 21:24
  Speaker 1: we're starting to do that. And that's kind of my expertise.
• 21:27
  Speaker 4: Cool.
• 21:28
  Speaker 3: And so is that all of the layers then?
• 21:30
  Speaker 1: No, then the outer layer measures the energy of the particles.
• 21:33
  Speaker 1: So the inner layer tries to figure out where the
• 21:35
  Speaker 1: particles went to give you like a visual picture of
• 21:37
  Speaker 1: the trajectories without affecting their angles, because you want to
• 21:41
  Speaker 1: measure their momentum. You don't want to change their direction.
• 21:43
  Speaker 1: So it's very lightweight. The outer layer of the detector
• 21:46
  Speaker 1: is the opposite. It tries to stop the particles. So
• 21:49
  Speaker 1: it forces them to interact and just dump their energy.
• 21:53
  Speaker 1: So we call that a calorimeter because it just measures
• 21:55
  Speaker 1: their energy and its layers of thick material that have
• 21:59
  Speaker 1: lots of protons in them and lots of neutrons in
• 22:01
  Speaker 1: them so that they'll slam into them. And even neutrons
• 22:04
  Speaker 1: will lose their energy there. They create a little shower
• 22:06
  Speaker 1: by hitting one particle, which hits another particle, which hits
• 22:09
  Speaker 1: another particle. And so all their energy gets dumped into
• 22:12
  Speaker 1: these layers and we can measure how much energy was deposited.
• 22:16
  Speaker 1: And so that creates like a little splash of energy
• 22:18
  Speaker 1: and we can tell this particle had this much energy.
• 22:21
  Speaker 1: So those are the two basic layers of the detector.
• 22:24
  Speaker 1: And that lets us figure out what was what. For example,
• 22:27
  Speaker 1: if you have an electron, It'll leave a trace through
• 22:30
  Speaker 1: your inner detector because it's charged, and then a splash
• 22:33
  Speaker 1: in the outer detector that looks like a little charged particle.
• 22:36
  Speaker 1: A photon will fly through your inner detector and leave
• 22:39
  Speaker 1: no trace because it doesn't have a charge. But then
• 22:42
  Speaker 1: it will make a splash in the outer detector that
• 22:44
  Speaker 1: looks just like an electron. So the difference between an
• 22:47
  Speaker 1: electron and a photon in our detector is, does it
• 22:50
  Speaker 1: have a trace in the inner detector or not? And
• 22:53
  Speaker 1: muons look different because they interact with the outer layer
• 22:56
  Speaker 1: a little bit differently, so they leave a smaller trace.
• 22:59
  Speaker 1: So with all these clues, we can figure out what
• 23:01
  Speaker 1: was there, what was going on. Some particles are really
• 23:04
  Speaker 1: challenging because they leave no information at all. Like a
• 23:07
  Speaker 1: neutrino does not interact with our detector. It's just gone.
• 23:12
  Speaker 1: So we have to deduce the presence of invisible particles
• 23:15
  Speaker 1: by looking at the balance. Like the beams come in
• 23:18
  Speaker 1: along the beam pipe. And so anything that shoots out
• 23:20
  Speaker 1: the side needs something shooting out the other side to
• 23:23
  Speaker 1: balance its momentum. So if you only see stuff on
• 23:26
  Speaker 1: one side, you know there was something invisible on the
• 23:28
  Speaker 1: other side. So we can deduce the existence of invisible
• 23:32
  Speaker 1: particles that way. And that's, for example, how we look
• 23:35
  Speaker 1: for dark matter. And so it's big, it's complicated, it's really,
• 23:39
  Speaker 1: really fun. And it's extra complicated by the fact that
• 23:41
  Speaker 1: these bunches come every 25 nanoseconds. <HAL> <HAL> <HAL> <HAL> <HAL> <HAL> <HAL> <HAL> <HAL> <HAL> <HAL> <HAL>
• 23:47
  Speaker 1: Oh my gosh. And every bunch has multiple collisions. So
• 23:51
  Speaker 1: sometimes you get like 200 collisions all on top of
• 23:54
  Speaker 1: each other, filling your detector up. with these signals and
• 23:57
  Speaker 1: it's really hard to disentangle them. It's good to get
• 24:00
  Speaker 1: lots of bunches because you have a higher chance to
• 24:02
  Speaker 1: see rare stuff and weird stuff you're looking for, but
• 24:05
  Speaker 1: it's harder to analyze because they're all on top of
• 24:07
  Speaker 1: each other. And sometimes something will take a while to
• 24:11
  Speaker 1: drift to the detector and will leave energy that deposits
• 24:14
  Speaker 1: across several bunches. And that's extra complicated because it's out
• 24:18
  Speaker 1: of time. And so, you know, everything is happening as
• 24:21
  Speaker 1: fast as possible. to get as many collisions as possible,
• 24:25
  Speaker 1: to look for the weirdest, rarest stuff, but that makes
• 24:28
  Speaker 1: it harder to understand the data.
• 24:30
  Speaker 2: So this system is like absolutely amazing and incredibly impressive,
• 24:34
  Speaker 2: like human ingenuity, wow. But if you were magic and
• 24:38
  Speaker 2: you could have one extra layer in there, is there
• 24:41
  Speaker 2: something else you would measure if we knew how to
• 24:44
  Speaker 2: do it or could afford to do it? Or is
• 24:45
  Speaker 2: this everything you could possibly want?
• 24:48
  Speaker 1: There's always more angles you can use by getting more information. Like,
• 24:53
  Speaker 1: for example, sometimes you can add timing information to these detectors. Like,
• 24:58
  Speaker 1: you can tell exactly when it was hit. So you
• 25:01
  Speaker 1: can deduce, like, how long it took the particle to
• 25:04
  Speaker 1: fly from here to there. And you can tell, like,
• 25:06
  Speaker 1: is it out of time? Is it from another bunch?
• 25:09
  Speaker 1: Or in the tracking detectors, sometimes you can do more
• 25:12
  Speaker 1: than just say, was there a particle here? You can measure, like,
• 25:16
  Speaker 1: how much energy deposited, like, per millimeter. And that can
• 25:20
  Speaker 1: help you discriminate between different kinds of particles. some of
• 25:23
  Speaker 1: which are ionized more strongly and some of which ionize
• 25:26
  Speaker 1: less strongly, or new weird models of particles with strange interactions.
• 25:31
  Speaker 1: And so there's always more depth that you could add
• 25:34
  Speaker 1: to these things if you have the money to build
• 25:36
  Speaker 1: more instrumentation. But I think the most powerful thing is
• 25:39
  Speaker 1: just more layers. If we had more money, we could
• 25:43
  Speaker 1: have more layers in these detectors, it would give us
• 25:45
  Speaker 1: a clearer picture of what's going on and helping us disentangle.
• 25:49
  Speaker 1: Like if you have a hundred layers, you do a
• 25:51
  Speaker 1: better job than if you have 10 layers of figuring
• 25:53
  Speaker 1: out like which particle was going where. So that's where
• 25:57
  Speaker 1: I would put the money.
• 25:57
  Speaker 2: So Okay, our third question of the day is from Mark,
• 26:21
  Speaker 2: and Mark is asking about something I have never heard
• 26:24
  Speaker 2: of before.
• 26:26
  Speaker 1: Hi, Dr.
• 26:27
  Speaker 4: Dan. I'm just a little perplexed about the sitter space.
• 26:31
  Speaker 3: I don't kind of get it. Would you be able
• 26:33
  Speaker 3: to explain it to me as though I'm a five-year-old?
• 26:35
  Speaker 4: Thank you.
• 26:37
  Speaker 2: Okay, Daniel, is De Sitter Space the place where the
• 26:39
  Speaker 2: babysitter tends to sit when they come to watch your kids?
• 26:44
  Speaker 1: De Sitter Space is named after a very smart dude,
• 26:47
  Speaker 1: Willem De Sitter, who died in 1934. He was a
• 26:51
  Speaker 1: professor in Leiden. And he worked closely with Albert Einstein
• 26:55
  Speaker 1: and thought a lot about the universe and the shape
• 26:57
  Speaker 1: of the universe and the nature of space itself. And
• 27:00
  Speaker 1: he came up with sort of a really useful thought
• 27:02
  Speaker 1: experiment for how to think about the universe in a
• 27:05
  Speaker 1: clear way. And that's called de-sitter space. And so we
• 27:10
  Speaker 1: have this tradition in physics of naming things after the
• 27:13
  Speaker 1: people who came up with them. And I think it's
• 27:16
  Speaker 1: a little awkward when that person's name starts with one
• 27:20
  Speaker 1: of these like prepositions, you know, like de or of, or,
• 27:23
  Speaker 1: you know, there's a guy in the Netherlands, his name
• 27:26
  Speaker 1: is tehoeft, you know, T apostrophe. But, you know, that's
• 27:30
  Speaker 1: what happened. So we call it de-sitter space.
• 27:33
  Speaker 2: So when you come up with something, do you just
• 27:34
  Speaker 2: have to hope that people will go ahead and name
• 27:37
  Speaker 2: it after you and they won't, like, miss that opportunity?
• 27:39
  Speaker 2: Or do you go ahead in your paper and say,
• 27:41
  Speaker 2: let's go ahead and call it DeSitter Space. Don't miss
• 27:43
  Speaker 2: that I came up with this.
• 27:44
  Speaker 1: You don't call it that yourself. You hope that people
• 27:46
  Speaker 1: refer to it. If it's a really useful idea, they'll
• 27:49
  Speaker 1: refer to it later as, you know, the Wienersmith model or,
• 27:54
  Speaker 1: you know, the Goldstone boson or whatever it is. Yang-Mills theory.
• 27:58
  Speaker 1: This is coined by other people who refer to it.
• 28:01
  Speaker 1: And this is sort of controversial. You know, like the
• 28:04
  Speaker 1: reason the Higgs boson is called the Higgs boson is
• 28:07
  Speaker 1: because Steven Weinberg started referring to it that way in
• 28:10
  Speaker 1: his papers because he just looked at the papers and
• 28:13
  Speaker 1: which ones arrived first at the journal first. And he thought, well,
• 28:17
  Speaker 1: that's the end of the story. Higgs paper arrived first
• 28:19
  Speaker 1: at the journal. But actually, he was reading the order
• 28:22
  Speaker 1: of the dates of publication and other papers arrived first.
• 28:26
  Speaker 1: And there was a team of young postdocs whose paper
• 28:29
  Speaker 1: arrived later, but was done independently. And they were nice
• 28:32
  Speaker 1: and they cited Higgs just like to be kind, even
• 28:35
  Speaker 1: though their work was totally independent. And so folks like
• 28:38
  Speaker 1: Weinberg thought that their work must be derivative because they
• 28:42
  Speaker 1: cited Higgs and didn't give them any credit. And they
• 28:44
  Speaker 1: were actually cut out of the Nobel prize 50 years
• 28:46
  Speaker 1: later because of that. I'm not blaming Weinberg. It's like
• 28:50
  Speaker 1: an innocent mistake, but this stuff is important. Yeah.
• 28:53
  Speaker 4: Okay.
• 28:54
  Speaker 1: Down the road.
• 28:56
  Speaker 4: All right.
• 28:56
  Speaker 3: So we've got a thing called de Sitter space. What
• 28:58
  Speaker 3: is de Sitter space?
• 29:00
  Speaker 4: All right.
• 29:00
  Speaker 1: So we live in a complicated universe because we have
• 29:03
  Speaker 1: matter and we have radiation and And matter and radiation,
• 29:07
  Speaker 1: what does it do to the universe? It tends to
• 29:09
  Speaker 1: collapse it, right? Like it pulls things together. And if
• 29:13
  Speaker 1: we had more matter in the universe, it would collapse
• 29:15
  Speaker 1: the universe more. We also have things like dark energy.
• 29:19
  Speaker 1: Dark energy is accelerating the expansion of the universe. It's
• 29:22
  Speaker 1: pushing stuff apart. So these two things are working against
• 29:25
  Speaker 1: each other. Dark energy is expanding space. Matter is collapsing space.
• 29:30
  Speaker 1: So that's complicated. And De Sider thought, well, let's think
• 29:33
  Speaker 1: of a simpler universe. What would happen if you had
• 29:36
  Speaker 1: a universe with no matter in it at all? No photons,
• 29:39
  Speaker 1: no electrons. nothing except for dark energy. So at the time,
• 29:44
  Speaker 1: they called that a positive cosmological constant, something to expand
• 29:48
  Speaker 1: the universe. And the interesting thing about that, even though
• 29:52
  Speaker 1: it sounds kind of boring because you've got like no
• 29:54
  Speaker 1: cats and no lava and nothing, is that in that universe,
• 29:58
  Speaker 1: space still expands.
• 30:00
  Speaker 3: Okay.
• 30:00
  Speaker 1: And so the point is empty space is not really empty.
• 30:05
  Speaker 1: It has this cosmological constant, has dark energy. And so
• 30:08
  Speaker 1: it can still do stuff. Empty space can still expand,
• 30:13
  Speaker 1: which is like, it's weird enough to think about the
• 30:15
  Speaker 1: universe expanding by like making new space between galaxies and
• 30:20
  Speaker 1: Now we're talking about the universe expanding and there's nothing
• 30:23
  Speaker 1: in it. And still it's getting bigger, which is like, okay,
• 30:28
  Speaker 1: the space between what are you talking about? Because we
• 30:31
  Speaker 1: say on the show a lot, and I think it's true,
• 30:33
  Speaker 1: that space is just the distances between stuff, right? You
• 30:36
  Speaker 1: have pairs of objects and you measure the distance between
• 30:39
  Speaker 1: the stuff. That's what space is. It's just a list
• 30:42
  Speaker 1: of distances between objects. So in a universe with nothing
• 30:46
  Speaker 1: in it, what does it mean to have distances? And
• 30:49
  Speaker 1: so There's a bit of a technical distinction here. We
• 30:52
  Speaker 1: say the universe has nothing in it, but when we
• 30:53
  Speaker 1: talk about distances, we imagine two observers in the universe
• 30:58
  Speaker 1: and measuring the distance between them. So when we talk
• 31:01
  Speaker 1: about disorder space, we don't literally mean a totally empty universe.
• 31:04
  Speaker 1: We mean a universe with nothing in it except for
• 31:07
  Speaker 1: two observers who are measuring the distance between them in
• 31:11
  Speaker 1: free fall. And if they just sit in the universe
• 31:14
  Speaker 1: and it has nothing in it but dark energy, they
• 31:16
  Speaker 1: will notice space expanding between them.
• 31:20
  Speaker 4: Okay.
• 31:20
  Speaker 3: So dark energy is all you need for expansion.
• 31:23
  Speaker 1: Dark energy is all you need for expansion. Exactly. And
• 31:26
  Speaker 1: that's the point of De Sitter Space. You strip it
• 31:28
  Speaker 1: all down, remove the matter, remove the radiation, and what
• 31:32
  Speaker 1: you get is De Sitter Space. So this is named
• 31:35
  Speaker 1: after William De Sitter, who thought about this solution in
• 31:37
  Speaker 1: the early 20th century. And they were trying to understand
• 31:42
  Speaker 1: what is the vacuum? What is space? What is the universe?
• 31:44
  Speaker 1: How does it work? What do you need? And a
• 31:47
  Speaker 1: common trick in physics is think about the extreme version
• 31:50
  Speaker 1: because often that's easier to calculate and it tells you something.
• 31:54
  Speaker 1: It tells you about the direction of the solutions, right?
• 31:57
  Speaker 1: Like that's what a black hole is. It's an extreme
• 31:59
  Speaker 1: version of gravity. This is the opposite of that. It's
• 32:02
  Speaker 1: like take away all the matter. What happens? Oh, interesting.
• 32:05
  Speaker 1: It still expands, right? And the expansion still accelerates. And
• 32:11
  Speaker 1: so it's sort of like your classic raisins and a bread,
• 32:15
  Speaker 1: except there's no raisins and there's no bread, but it's
• 32:19
  Speaker 1: still expanding and expansion still continues indefinitely.
• 32:23
  Speaker 2: And was this something that we would not have realized
• 32:26
  Speaker 2: without this thought exercise? I mean, I guess that must
• 32:29
  Speaker 2: be why it got a name is because this was
• 32:31
  Speaker 2: not something we had realized until he set out this
• 32:33
  Speaker 2: thought experiment.
• 32:34
  Speaker 1: Yeah, this is very early in general relativity, right? We
• 32:38
  Speaker 1: are sort of familiar with this. We're comfortable with the idea.
• 32:40
  Speaker 1: We've even observed this expansion. This is many decades before
• 32:45
  Speaker 1: we observed the accelerating expansion of the universe. This is
• 32:48
  Speaker 1: back when people were still playing around with general relativity
• 32:50
  Speaker 1: and trying to understand why. what does it predict? And thinking,
• 32:53
  Speaker 1: does that make any sense? Remember earlier, Einstein, when he
• 32:57
  Speaker 1: was developing his theory of general relativity, he thought the
• 32:59
  Speaker 1: universe was static. And so his solution suggested, oh, if
• 33:03
  Speaker 1: you just have a bunch of mass in the universe,
• 33:05
  Speaker 1: it'll just collapse. And our universe hasn't collapsed. So he
• 33:08
  Speaker 1: was like, hmm, that's weird. So he added in basically
• 33:11
  Speaker 1: dark energy to balance that, to make the universe static.
• 33:15
  Speaker 1: Then later we discovered, oh, the universe is not static.
• 33:18
  Speaker 1: It's expanding. So he got rid of dark energy and
• 33:20
  Speaker 1: just thought, while the universe is shooting out in every
• 33:23
  Speaker 1: direction and it is collapsing, it just hasn't collapsed yet.
• 33:27
  Speaker 1: And then later we discovered, oh no, it's not collapsing
• 33:30
  Speaker 1: at all. It's accelerating. And so to put the dark
• 33:32
  Speaker 1: energy back in and say, okay, it's expanding and there's
• 33:37
  Speaker 1: this force increasing the expansion.
• 33:39
  Speaker 3: Okay. Got it.
• 33:40
  Speaker 1: So this is very early on. People are trying to
• 33:42
  Speaker 1: understand what this means. And when you create a universe
• 33:45
  Speaker 1: like this, that's filled with nothing except for dark energy,
• 33:48
  Speaker 1: there's still weird stuff that happens and that's insightful. Like
• 33:52
  Speaker 1: there's still a horizon. Like because of the accelerated expansion
• 33:56
  Speaker 1: of the universe, there are parts of space where the
• 33:58
  Speaker 1: expansion means we will never see them. Like a photon
• 34:02
  Speaker 1: that's shot out from far enough away will never get
• 34:05
  Speaker 1: here because the expansion of space between us and the
• 34:09
  Speaker 1: thing that shot out the photon is happening faster than
• 34:11
  Speaker 1: the speed of light. Basically space is being created Faster
• 34:15
  Speaker 1: than the photon is going through it.
• 34:16
  Speaker 4: Whoa.
• 34:16
  Speaker 1: So it'll just never get here, right? It's like somebody's
• 34:19
  Speaker 1: creating new track in front of it constantly. And so
• 34:22
  Speaker 1: it's never going to get to where it's going.
• 34:24
  Speaker 4: Wow.
• 34:24
  Speaker 1: And that happens even in an empty universe. And so
• 34:28
  Speaker 1: you have this cosmic horizon in desider space, which is
• 34:32
  Speaker 1: weird to think about because the universe is empty. But
• 34:36
  Speaker 1: it does lead to fascinating consequences, right? Like later on,
• 34:39
  Speaker 1: people were thinking about desider space and they were like, huh,
• 34:43
  Speaker 1: you have a horizon. How does that interact with quantum mechanics?
• 34:47
  Speaker 1: Because we know that if you have a black hole
• 34:49
  Speaker 1: event horizon, that that generates radiation. Because quantum mechanical fields
• 34:55
  Speaker 1: near an event horizon will generate radiation. That's Hawking radiation.
• 34:59
  Speaker 1: And so people have done calculations and realized that this
• 35:03
  Speaker 1: kind of horizon, the cosmic horizon in the center space,
• 35:07
  Speaker 1: also interacts with quantum fields in the same way and
• 35:09
  Speaker 1: also generates radiation. So if you are an observer in
• 35:14
  Speaker 1: an expanding universe in desider space, you will see radiation
• 35:20
  Speaker 1: from the cosmic horizon. So they call this a thermal bath.
• 35:24
  Speaker 1: They will just be like photons and other particles created
• 35:27
  Speaker 1: from this horizon. And we've dug into like, what is
• 35:31
  Speaker 1: Hawking radiation? What is it not? How does it work
• 35:33
  Speaker 1: another time? For now, just remember that having a horizon
• 35:37
  Speaker 1: and having quantum mechanics means radiation. And so that happens
• 35:41
  Speaker 1: even in an empty universe.
• 35:43
  Speaker 4: Wow.
• 35:43
  Speaker 1: De Sitter space, you see this radiation, which is kind
• 35:47
  Speaker 1: of cool.
• 35:48
  Speaker 3: That is kind of cool.
• 35:49
  Speaker 1: Yeah. So basically it provides like, you know, a laboratory
• 35:52
  Speaker 1: for thinking about horizons and quantum fields and gravity and
• 35:55
  Speaker 1: what is empty space anyway. And the cool thing is
• 35:59
  Speaker 1: that our universe is becoming more De Sitter like every day.
• 36:03
  Speaker 3: What does that mean?
• 36:04
  Speaker 1: Well, De Sitter space is a universe that's just dark energy.
• 36:08
  Speaker 1: Everything else is gone. And our universe is heading that
• 36:11
  Speaker 1: way because as the universe expands, dark energy does not dilute.
• 36:17
  Speaker 1: You make new space, it comes with more dark energy.
• 36:20
  Speaker 1: But matter does get diluted. You make new space, you
• 36:22
  Speaker 1: don't make new protons and electrons. Proton energy density goes down.
• 36:27
  Speaker 1: And new space means more dark energy, which means more space,
• 36:31
  Speaker 1: which means more dark energy. So the dark energy fraction
• 36:33
  Speaker 1: of the universe is going up. Around 6 billion years ago,
• 36:37
  Speaker 1: it began to dominate. And now it's even more dominant.
• 36:41
  Speaker 1: And in another 10 billion years, it'll be even more dominant.
• 36:44
  Speaker 1: And in a trillion years, the universe will be 99.99
• 36:47
  Speaker 1: whatever percent dark energy. And the sitter space is a
• 36:52
  Speaker 1: universe that's 100% dark energy. So we're moving towards the
• 36:55
  Speaker 1: sitter space, basically, as the universe gets more and more
• 36:58
  Speaker 1: spread out. And we transition to like a bunch of
• 37:01
  Speaker 1: huge black holes separated by vast expanding distances. That's like
• 37:07
  Speaker 1: almost De Sitter space. It's De Sitter space with a
• 37:09
  Speaker 1: bunch of big black holes scattered through it. They can't
• 37:13
  Speaker 1: really do anything.
• 37:14
  Speaker 4: Wow.
• 37:14
  Speaker 3: So, okay.
• 37:15
  Speaker 2: So if dark energy is all you need to create
• 37:17
  Speaker 2: an expanding universe, when we get more dark energy, does
• 37:20
  Speaker 2: that mean we should predict that the universe will keep
• 37:23
  Speaker 2: accelerating and maybe it'll keep speeding up and we'll get
• 37:27
  Speaker 2: more radiation at the horizon over time?
• 37:31
  Speaker 1: Yes, but that radiation is really, really tiny, like absurdly
• 37:34
  Speaker 1: tiny because dark energy is not that dense, right? It's
• 37:38
  Speaker 1: a very small amount of dark energy per unit of space.
• 37:41
  Speaker 1: So I don't know that we'll ever be able to
• 37:42
  Speaker 1: see that. The other interesting thing about De Sitter space
• 37:45
  Speaker 1: is that there's the opposite thought experiment as well. So
• 37:49
  Speaker 1: dark energy creates positive curvature. It's expanding the universe. You
• 37:54
  Speaker 1: can also imagine a universe where you have nothing but
• 37:57
  Speaker 1: negative curvature. So the universe is collapsing. And so that's called.
• 38:01
  Speaker 3: Is this going to be... De-stander space?
• 38:03
  Speaker 1: I'm going to take a nap space. Yeah, exactly. Nice, nice. No,
• 38:11
  Speaker 1: they're not that clever. They call it anti-de-sitter space.
• 38:13
  Speaker 3: Oh, okay.
• 38:14
  Speaker 1: It's the opposite. And if you hear about holography and
• 38:18
  Speaker 1: something called ADS-CFT, which is a big deal in string
• 38:23
  Speaker 1: theory and fundamental physics and a huge step forward by
• 38:26
  Speaker 1: Juan Maldicena, something like 20 years ago, because it connects
• 38:30
  Speaker 1: quantum mechanics and gravity. It shows you that some theories
• 38:34
  Speaker 1: of gravity which live in anti-de Sitter space are the
• 38:38
  Speaker 1: same as theories of quantum mechanics in conformal field theories. ADS,
• 38:44
  Speaker 1: anti-de Sitter space, CFT, conformal field theories. A big result
• 38:48
  Speaker 1: in quantum gravity because it showed for the first time
• 38:51
  Speaker 1: that there are some gravitational theories that are very similar
• 38:53
  Speaker 1: to quantum theories. That's cool. But that's what ADS stands
• 38:57
  Speaker 1: for in ADS CFT. And The big fly in the
• 39:01
  Speaker 1: ointment is that it only works for anti-de-sitter space, right?
• 39:05
  Speaker 1: Whereas we live in a de-sitter universe. So the whole
• 39:09
  Speaker 1: quantum gravity step forward of ADS-CFT is relevant to the
• 39:13
  Speaker 1: opposite kind of universe than the one that we live in. Which,
• 39:17
  Speaker 1: you know, doesn't mean one day somebody won't figure out
• 39:19
  Speaker 1: how to do it in de-sitter space instead of anti-de-sitter space. But,
• 39:24
  Speaker 1: you know, it means it's further from being relevant.
• 39:27
  Speaker 3: All right. All right, Mark. Find de-space. to sit down
• 39:31
  Speaker 3: and let us know what you think of this answer.
• 39:34
  Speaker 2: Wow.
• 39:35
  Speaker 3: Thanks, Dan and Kelly. What a concept.
• 39:39
  Speaker 4: I think I've got a bit more of a handle
• 39:42
  Speaker 4: on it now. Thanks very much. All right.
• 39:45
  Speaker 1: Thank you very much, everybody, for sending in those questions.
• 39:48
  Speaker 1: I love hearing from you. I love your questions. Please
• 39:51
  Speaker 1: write to us, questions at danielandkelly.org. It literally makes our day.
• 39:56
  Speaker 3: It does. Until next time.
• 40:04
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• 40:12
  Speaker 2: Daniel and Kelly's Extraordinary Universe is edited by the amazing
• 40:16
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• 40:17
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